NEC830 deals with requirements for broadband communication systems, such as cable television systems, which provide powering on, or attached to, the coaxial cable on which signals are transmitted to bring, for example, television programs and other services to subscribers. Coaxial cable usually runs from a device called a xe2x80x9ctap,xe2x80x9d which extracts a portion of the power from the distribution coaxial cable, to a subscriber""s building or residence, hereinafter referred to simply as xe2x80x9cthe subscriberxe2x80x9d. This coaxial cable is called the xe2x80x9cdropxe2x80x9d. NEC830 requires that if the drop carries power above a certain voltage, it must meet certain requirements for the protection of people who may come in contact with it. The most common reason the drop would be carrying power is to supply a cable telephone interface box, or network interface device (NID). The requirements of NEC830 are that the drop must meet certain physical requirements, or it must be protected by a UL xe2x80x9clisted fault protection devicexe2x80x9d, to detect if a drop is either shorted or open and, in either case, interrupt power carried in the drop until the fault has cleared.
Broadband services, such as cable telephony applications, may require AC power to be supplied from the tap cable to the drop cable. The coaxial cable drop to the subscriber is therefore xe2x80x9cAC hotxe2x80x9d and subjects the installer or other craftspersons working with the cable drop to potential electrical shock and/or electrocution.
An example of an NEC830 safety circuit for interrupting AC power on the drop cable is described in U.S. Pat. No. 5,793,590 to Vokey et al. The device described in Vokey is a two-unit device having a tap unit and a premise unit. An independent DC power source is used to power the two-unit device and to provide a probe voltage. A dual threshold detector compares a DC voltage drop proportional to the AC power transferred to the drop cable with a probe voltage to determine whether there is a short or open condition at the junction of the tap and drop cables. While Vokey is suitable for its general purpose, it utilizes an analog methodology that can lack long-term calibration dependability in harsh environmental conditions. Also, the limitations of monitoring a DC voltage drop proportional to the AC power transferred to the drop cable tends to make the Vokey methodology inaccurate because the monitored DC voltage drop is merely an approximation of the actual AC power transferred to the drop cable. In addition, the second (DC) power source creates certain inefficiencies. A capacitor must keep the DC power source separate from the AC power signals in the drop cable and thus is critical for the operation of the safety device as a whole. This capacitor""s necessarily large physical size makes it cumbersome for circuit packaging. Capacitors of this size are available, but along with being larger than one would like in this application, getting one with an adequate current rating and good long term reliability in an outdoor (but sheltered) environment is difficult. Thus, for an efficient and reliable safety device solution it would be desirable to eliminate the need for large value capacitors and to have a precise, rather than approximate, monitoring capability.
The present invention overcomes the above-described problems in the prior art by providing a coaxial cable safety device that utilizes the AC power already present in a drop cable as a powering and monitoring solution to provide a parameter indicative of such AC power. The present invention is a coaxial cable protection system for a cable media environment that includes a primary cable and a drop cable having first and second conductors. The first and second conductors carry AC power and RF signals to a network interface device. The present invention includes a first circuit component for allowing the RF signals to be transmitted from the primary cable to the first conductor of the drop cable. A second circuit component features an active state for passing the AC power from the primary cable to the first conductor of the drop cable and a blocking state for selectively blocking the passage of AC power to the first conductor. A sensor actuates the second circuit component to block AC power from the first conductor if the current drawn between the first and second conductors is outside of a prescribed range.
A variation of the present invention is a sensing circuit utilizing the resistance in a drop cable. An oscillator generates a probe frequency. A comparator compares the resistance in a conductor with a known resistance. The known resistance is proportional to the probe frequency. An actuator opens and closes a circuit component for blocking AC power from passing to the drop cable based on the comparison of the drop cable resistance provided by the comparator with the known resistance.
Another variation of the present invention uses a frequency generation and detection circuit that monitors the drop cable for a tone signal. The drop cable has a first end and a remote end. An oscillator utilizes the AC signal from the drop cable to generate a tone signal. A circuit component actuated by the oscillator will block AC power in the drop cable if the oscillator does not generate a tone signal and will allow the AC power to pass if oscillator generates a tone signal.